Ionization type smoke sensor

Abstract

An ionization-type smoke sensor having a 2-chamber 1-radiation source structure including an inner electrode having a radiation source, an intermediate electrode having an opening through which radiation from the radiation source passes, an outer electrode through which smoke may flow inside from outside, and an insulator supporting and insulating the inner electrode, the intermediate electrode and the outer electrode. In the sensor, a fence portion made of an insulating material is formed on and projecting from a surface of the insulator which faces the outer electrode, the fence portion surrounding the periphery of the opening of the intermediate electrode. By virtue of this arrangement, even when the sensor is subjected to flow of air heading from outside toward the opening of the intermediate electrode, the fence serves to reduce the flow of air heading toward the opening, thereby enabling the characteristics of the sensor chambers to remain stable against flow of air. Since the fence portion made of an insulating material is formed to project from the surface of the insulator, the creepage distance between the intermediate electrode and the outer electrode is long, thereby enabling an improved level of insulation for the electrodes.

What is claimed is: 
     
       1. An ionization-type smoke sensor comprising: a printed circuit board with electrical parts mounted thereon; an inner electrode electrically connected with said printed circuit board, said inner electrode having a radiation source; an intermediate electrode disposed axially outward of said inner electrode and defining an inner ionization chamber in cooperation with said inner electrode, said intermediate electrode having an opening through which radiation from said radiation source passes; an outer electrode disposed axially outward of said intermediate electrode and defining an outer ionization chamber in cooperation with said intermediate electrode, smoke from outside being flowable into said outer ionization chamber; and an insulator supporting and insulating said inner electrode, said intermediate electrode and said outer electrode, said ionization-type smoke sensor comprising further a fence portion formed on and projecting from a surface of said insulator facing said outer ionization chamber and surrounding the periphery of said opening of said intermediate electrode, said fence portion being made of an insulating material; said fence portion comprising a plurality of concentric fence members having a height increasing stepwise radially inward toward the center of said sensor.   
     
     
       2. An ionization-type smoke sensor according to claim 1, wherein said insulator is integral with said fence portion. 
     
     
       3. An ionization-type smoke sensor according to claim 1, wherein said fence portion is inclined radially inward toward the center of said sensor. 
     
     
       4. An ionization-type smoke sensor according to claim 1, wherein said fence portion is mesh-like.